Cellular communication systems are known. Such systems are, typically, comprised of a number of cells, each having a service coverage area, and a number of cellular telephones (communication units). The service coverage areas of adjacent cells may be arranged to partially overlap in such a manner as to provide a substantially continuous coverage area in which a communication unit receiving service from one cell may be handed off to an adjacent cell with no interruption in service. The Groupe Special Mobile (GSM) Pan-European digital cellular system, as specified in GSM recommendations available from the European Telecommunications Standards Institute (ETSI) and incorporated herein by reference, is an example of just such a system.
A cell's radio coverage is provided by a base transceiver station (BTS). Communication between a BTS and a mobile communication unit (or mobile station) (MS) typically occurs using a portion of a pair of frequencies (transmit and receive) temporarily assigned in support of the communication transaction at the BTS.
The pair of frequencies assigned for use at the remote site are typically referred to as a radio channel. Downlink transmissions (from BTS to MS) on the radio channel occur on a first frequency of the pair of frequencies. Uplink transmissions (from MS to BTS) on the radio channel occurs on the second frequency of the pair of frequencies.
The exchange of non-voice signals (e.g. facsimile or data) within the GSM system, between an MS and BTS, is also known. Such exchanges are well known and may be accomplished under well defined procedures (see GSM recommendation 3.45 for Group 3 facsimile transmissions involving MSs).
GSM recommendation 3.45 provides a means for allowing facsimile machines to interface with and transmit an information signal through the largely dissimilar, cellular network. dissimilarities addressed by the GSM recommendations range from the substantially different signalling protocols of fax machines designed for use on PSTN channels versus cellular networks involving error prone air interfaces to the analog nature of a fax signal versus the digital channels provided by GSM.
Under GSM recommendations, signalling protocols of a fax machine are converted to a GSM compatible format by a fax adapter that interconnects the fax to the mobile. In the case where a mobile fax interfaces with a PSTN fax, a protocol converter is also required at the cellular/PSTN interface. Whereas a fax machine is designed to operate via PSTN channels, a GSM specified fax adapter facilitates the same to utilize communication channels provided by GSM cellular networks.
Once a fax call is established then the line-by-line transmission of facsimile data may begin. Under CCITT recommendation T.4, a faxed line may be transmitted in as little as 5 ms or as long as 5 seconds. If the actual time of transmission takes less than 5 ms then filler bits may be added to increase the time to 5 ms. If the time is longer than 5 seconds then the receiving fax may determine that a fault exists and disconnect.
While the procedures established by GSM recommendation 3.45 may work well, problems arise during handoff of an MS among BTSs. During handoff of an MS between BTSs, signal interruptions of typically 200 ms are common. If a fax is being transmitted, and a handoff should occur then as much as 40 lines of the faxed message may be lost. Information may be lost upon handoff during the image transfer phase of a fax call because no provision is made within GSM recommendation 3.45 to avoid data loss due to handoff. Because of the importance of facsimile transmission of documents a need exists for a method of avoiding loss of data during a cellular handoff.